在PostgreSQL

Question

WITH Dept(DName) AS (
    VALUES ('D1'), ('D2'), ('D3') 
) , 
Emp(DName, EName, Age, Loc) AS (
    VALUES ('D1','E11',20,'L1'), ('D1','E12',25,'L2'),('D2','E21',28,'L1') 
) 
SELECT DName, 

    (SELECT avg(Age) 
    FROM Emp 
    WHERE Loc = 'L1' 
    AND DName = d.DName) AS "L1 Avg", 

    (SELECT avg(Age) 
    FROM Emp 
    WHERE Loc = 'L2' 
    AND DName = d.DName) AS "L2 Avg" 
FROM Dept d 
LEFT JOIN Emp USING (DName) 
GROUP BY DName 

刪除子查詢這個查詢的輸出是：

"D1";20.0000000000000000;25.0000000000000000 
"D2";28.0000000000000000; 
"D3";; 

有沒有辦法通過刪除子查詢重構查詢與和PostgreSQL中更好地構建更換呢？

Answer 1

SELECT DName, 
     avg(case when loc = 'L1' then age else null end) as "L1 Avg", 
     avg(case when loc = 'L2' then age else null end) as "L2 Avg" 
FROM Dept d 
LEFT JOIN Emp USING (DName) 
GROUP BY DName